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The effect of geometric and electric constraints on the
performance of polymer-stabilized cholesteric liquid
crystals with a double-handed circularly polarized light
reflection band
Sabrina Relaix, Michel Mitov
To cite this version:
Sabrina Relaix, Michel Mitov. The effect of geometric and electric constraints on the performance
of polymer-stabilized cholesteric liquid crystals with a double-handed circularly polarized light
re-flection band. Journal of Applied Physics, American Institute of Physics, 2008, 104 (3), pp.033539.
�10.1063/1.2968245�. �hal-02887327�
The effect of geometric and electric constraints on the performance
of polymer-stabilized cholesteric liquid crystals with a double-handed
circularly polarized light reflection band
Sabrina Relaixa兲and Michel Mitovb兲
Centre d’Elaboration de Matériaux et d’Etudes Structurales, CEMES, CNRS, Univ. Toulouse, BP 94347, F-31055 Toulouse Cedex 4, France
共Received 4 April 2008; accepted 12 June 2008; published online 13 August 2008兲
Polymer-stabilized cholesteric liquid crystals共PSCLCs兲 with a double-handed circularly polarized reflection band are fabricated. The geometric and electric constraints appear to be relevant parameters in obtaining a single-layer CLC structure with a clear-cut double-handed circularly polarized reflection band since light scattering phenomena can alter the reflection properties when the PSCLC is cooled from the elaboration temperature to the operating one. A compromise needs to be found between the LC molecule populations, which are bound to the polymer network due to strong surface effects or not. Besides, a monodomain texture is preserved if the PSCLC is subjected to an electric field at the same time as the thermal process intrinsic to the elaboration process. As a consequence, the light scattering is reduced and both kinds of circularly polarized reflected light beams are put in evidence. Related potential applications are smart reflective windows for the solar light management or reflective polarizer-free displays with higher brightness. © 2008 American
Institute of Physics.关DOI:10.1063/1.2968245兴
I. INTRODUCTION
Polymer-stabilized liquid crystals 共PSLCs兲 are two-component composite materials made by photopolymeriza-tion and photocrosslinking of liquid crystalline monomers dissolved in a low molecular weight LC.1 As the reaction proceeds, the reactive material tends to phase separate from the LC, leading to the formation of a polymer network with a very high surface area. The cholesteric LC 共CLC兲 helical structure results from macroscopic rotation caused by mo-lecular chirality. When aligned into the so-called planar tex-ture, it is distinguished by Bragg reflection of a circularly polarized light beam of the same handedness as the helix.2,3 Both the central position0 and the width⌬ of the reflec-tion band are determined by the pitch p of the helical struc-ture, ⌬=共n储− n⬜兲p and 0= np, where n储 and n⬜ are the
local values of the refractive indices of the CLC and n =共n储 + n⬜兲/2 is the average refractive index. Modulating the char-acteristics of the reflection band 共tuning 0in the visible or
infrared spectrum, broadening⌬, or increasing the reflected light flux兲 is a practical goal that has driven numerous re-search efforts related to applications such as polarizer-free reflective displays4共with no backlight requirement兲, polariz-ers and color filtpolariz-ers,5mirrorless lasing,6 or smart switchable reflective windows for the dynamical control of solar light.7 Since circularly polarized light of only one handedness is reflected by a CLC structure, the maximum reflection of am-bient 共unpolarized兲 light from a single-layer CLC is never greater than 50% at normal incidence. We have shown that this theoretical limit may be exceeded in a PSCLC single-layer material.8,9A low molecular weight CLC, which
exhib-its a thermally induced inversion of helicity at the tempera-ture TC, was blended with nematic diacrylate monomers. The blend was then cured with ultraviolet 共UV兲 light at a tem-perature T+⬎TCwhen the helix was right handed. Due to the memory effects brought by the polymer network, more than 50% of the unpolarized incident light was reflected by the PSCLC at a temperature T−⬍TC assigned to a CLC phase with the same pitch but a left-handed sense before reaction. The purpose of this paper, based on a different LC blend, is to report the crucial role of the confinement of the low molar LC in the porous polymer to allow the coexistence of both circularly polarized light components in the reflection band. Besides, the application of an electric field to the PSCLC, which has a negative dielectric anisotropy, preserves the re-flected light flux, and the measurement of both kinds of cir-cularly polarized reflected light beam is made possible. Since CLCs are used as tunable bandpass filters, reflectors, polar-izers, and temperature 共or pressure兲 sensors, novel opportu-nities to fabricate hyper-reflective displays and to modulate the reflection over the whole light flux range in a single-layer material are offered.
II. EXPERIMENT
A CLC oligomer共Wacker Chemie Ltd.兲 is blended with
the chiral low molecular weight LC 共S, S兲-EPHDBPE or
4-关共S, S兲-2, 3-epoxyhexyloxy兴-phenyl-4-共decyloxy兲-benzoate 共Sigma-Aldrich兲, which is the helix-inversion compound 共HIC兲. The oligomer is a cyclic polysiloxane molecule with two types of mesogen as lateral chains: a chiral共cholesterol group bearing兲 mesogen and an achiral 共biphenyl group bear-ing兲 mesogen.10
The pitch of the cholesteric phase, and as a consequence the associated reflection wavelength, depends on the number of chiral mesogens. Two types of oligomer were chosen: a photocrosslinkable oligomer, called RMR,
a兲Electronic mail: srelaix@lci.kent.edu. b兲Electronic mail: mitov@cemes.fr.
and a nonphotocrosslinkable oligomer, called SR共both mol-ecules have the same molar fraction of chiral mesogens, i.e., 31%兲. RMR and SR were mixed together in different propor-tions to change the concentration in a network-forming ma-terial. This two-component mixture was blended with the HIC with a ratio constant for all the investigations and equal to 12.5: 87.5 wt %. With such an experimental protocol, it is possible to change the concentration in the network-forming material in the three-component mixture without drastically changing the pitch of the cholesteric phase. For each blend, 2 wt % 共compared to the RMR content兲 of photoinitiator Irgacure 907共from Ciba–Geigy兲 is added for photocrosslink-ing purposes. The cholesteric structure is left共right兲 handed below共above兲 TC.
The mixture was introduced at 89 ° C by capillarity in a 25共⫾5兲 m thick indium tin oxide 共ITO兲-coated glass cell. Before curing, the cell was kept at T+= 98 ° C for a few
min-utes to favor the planar alignment of the blend. The sample
was then irradiated with UV light
共0.1 mW/cm2 at 365 nm兲 for 4 h. Finally, the cell was
cooled at 0.5 ° C/min until T−= 75 ° C.
The transmittance properties were investigated at normal incidence by infrared 共IR兲 spectroscopy between 1.9 and 4.5 m with a Perkin–Elmer IR spectrometer spectrum 100. The baseline was made when the blend was in the nematic state at TC. Measurements with circularly polarized incident light were performed using homemade circular polarizers fabricated by mixing two cyanobiphenyl CLC blends, BL094 and BL095 共from Merck Ltd.兲, for which the cholesteric phases exhibit the same pitch behavior but with opposite twist senses 共BL094 and BL095 are enantiomers兲. The re-flection wavelength as well as the twist sense of the choles-teric phase are tuned by varying their relative concentration: as a consequence, the kind of polarization is fixed. Due to the polarization selectivity rule for cholesteric reflectors, which are also polarizers, a left共right兲-handed helix gives rise to a
right 共left兲 circular polarizer. When measurements with cir-cularly polarized incident light were performed, a 25 m thick cell of pure BL094 共nonreflecting in the IR spectrum兲 is positioned between the source and the cell during the base-line step.
III. RESULTS AND DISCUSSION
A. Effect of the polymer network concentration on the temperature dependence of the reflection wavelength
The concentration in a network-forming material may determine the density of the network and the respective dis-tribution of free and bound fractions of LC molecules in the volume of the PSLC. Here the RMR concentration in the two-component blend ranges from 3.8 to 12.5 wt %. Figure 1 shows the variation in the selective reflection wavelength 0of the PSCLC as a function of temperature for the
differ-ent RMR concdiffer-entrations.
When the RMR concentration is low, between 3.8 and 5.0 wt %, the temperature behavior of 0 below as well as above TC is comparable to the behavior before curing; the influence of the polymer on the reflection properties is very weak. The pitch changes are hindered by the presence of oligomer molecules if the network has no tridimensional na-ture, and this situation is promoted for low concentrations. In that case, the well-aligned monodomain planar texture is bro-ken, and light scattering occurs, especially when the tem-perature is below TC. When the RMR concentration is be-tween 7.8 and 12.5 wt %, the0becomes independent of the
temperature and is kept between 2.7 and 2.9 m. The helical structure present at T+is frozen into the composite material.
The LC component undergoes strong surface interactions with the polymer network and has to be considered a bound fraction; the network is so dense that it hinders the reorgani-zation of LC molecules as the temperature is lowered.
Mid-2.5 2.7 2.9 3.1 3.3 3.5 3.7 3.9 4.1 4.3 4.5 70 75 80 85 90 95 100 Temperature (°C) Mean reflection wavelength 0 (µm) 3.8 wt.% - Tc = 84.9°C 5.0 wt.% - Tc = 84.4°C 6.2 wt.% - Tc = 84.0°C 7.8 wt.% - Tc = 83.6°C 12.5 wt.% - Tc = 82.5°C
FIG. 1. Mean reflection wavelength of the PSCLC as a function of temperature for different monomer concentrations. The critical temperature TCat which the helicity inversion occurs is mentioned for each case.
way between the two previously described behaviors, the case of a concentration such as 6.2 wt % is especially inter-esting, and we are going to focus the rest of the paper on this case. The 0 versus T curve is now bell shaped, which
ap-pears like the signature of an original intermediate memory effect, between the behaviors of the free fraction and the fully bound fraction. Indeed, a fraction of LC molecules be-haves as free molecules since there is a variation in the po-sition of the Bragg band with temperature: nevertheless this freedom is relative because 0 has limited values at each
temperature. The divergence is even not attained inside the temperature range around TC. In other words, the PSCLC material is still a reflector when its temperature is inside the temperature region in which the pitch diverged before cur-ing. The reflection behavior is dominated by the structural changes of LC molecules that might be considered as an interfacial population between the completely free and bound populations of LC molecules. The0slightly increases from 2.7 m at T+to about 3.1 m at 84 ° C, a temperature that
corresponds to the top of the bell-shaped curve. Below this value, the decrease in0, up to 2.7 m again at T−, can be
suspected of being the sign of a left-handed helicity, which is introduced in the structure of共at least a fraction of兲 LC mol-ecules. This can indirectly be put in evidence by transmit-tance measurements under circularly polarized incident light; the related investigations are reported in Sec. III B.
B. Impact of an electric field on the characteristics of the reflection band
When a thermal ramp starts from T+toward T−, the CLC
structure of the free fraction undergoes very drastic pitch changes, which even include unwinding of the helix at TC followed then by rewinding. Besides, these changes happen when the LC is strongly confined in a porous structure with a very large surface area. As a consequence, the formation of a well-aligned monodomain planar structure, which domi-nates at T+, is not possible anymore and the CLC slab has a tendency to spontaneously produce a polydomain texture when it is cooled. A phenomenon of light scattering arises from this situation, which we have discussed in a previous paper;11the slower the cooling rate, the smaller the transmis-sion losses are. The light scattering phenomenon is more present in the material of the present study than in the pre-vious one,11 in particular, because the confinement of LC molecules in the network is different. The confinement char-acteristics are mainly related to the nature of the polymer network: the flexibility of mesogens 共of paramount impor-tance兲, the concentration, the temperature, as well as the structure of the mesophase when the polymerization occurs. In close relation with the physical parameters of the blend, we propose here a solution to decrease the transmittance losses, which is an alternative to the decrease in the thermal ramp speed from T+to T−. Additionally, since measurements
of circularly polarized light with both helicities and their relative distributions are desired, it is required to reduce or minimize the light scattering phenomenon and to preserve a planarly well-oriented texture. This objective was achieved by subjecting the cell to an electric field when the tempera-ture was decreased from T+to T−and by taking advantage of
the negative dielectric anisotropy⌬ of the HIC. Indeed, an electric field applied to a CLC with a negative⌬ and in a direction parallel to the helix axis stabilizes and favors a homogeneous planar texture because it tends to align the small axis of the molecules in the direction of the field共if ⌬ was positive, the helix would be progressively unwound兲.12 Therefore, a monodomain texture is favored to the detriment of a polydomain one and, in return, the scattering of the light is not promoted. For illustration, Fig. 2 shows the optical micrographs of the cholesteric planar texture at the interface between the nonconducting and the共conducting兲 ITO-coated regions of the cell glass substrates, with and without field. The texture exhibits oily streak defects, which can be con-sidered like a network of defect lines dispersed in regions with a uniform helix direction;13 the structure of an oily streak mainly depends on elasticity and surface anchoring. Different red tints correspond to different twisted states共the number of pitch lengths varies from one region to the other one兲 due to hysteresis in temperature variation. It appears that the density and the thickness of oily streaks are greatly reduced when the CLC slab is subjected to an electric field. The network of oily streaks, which is generally not a static feature, is also modified very close to the interface between the nonconducting and conducting regions due to the edge effects.
Figure 3 shows the variation in the transmitted light losses due to the scattering as a function of temperature when an electric field 共91.5 V at 50 kHz兲 is applied or not. The losses are defined as the difference in light intensity trans-mitted by the PSCLC when measured out of the Bragg band at a wavelength arbitrarily chosen equal to 2 m, at T+, and at each measurement temperature T, i.e., I2m共T+兲
− I2m共T兲.
The losses increase when the temperature decreases but are clearly reduced when the electric field is applied during the thermal process. At the end共at T−= 75 ° C兲, the losses are
reduced by more than four times. The influence of the elec-tric field on the depth of the Bragg peak was also investi-FIG. 2.共Color online兲 Optical micrographs of the cholesteric planar texture at the interface between the nonconducting and the共conducting兲 ITO-coated regions of the glass substrates at T−= 74 ° C after thermal process:共a兲
with-out electric field and共b兲 with an electric field 共50 V at 1 kHz兲. Transmission mode. Polarizers are crossed. Scale bar= 100 m.
gated. Figure4shows the variation in the reflected intensity as a function of temperature, with and without field.
When the material is not subjected to the field, the re-flected light intensity progressively decreases by more than 10% when the temperature decreases. When the field is ap-plied, the reflected intensity is preserved except in the tem-perature range between TC共=84 °C兲 and 77 °C. This result is due to the decrease in0below 84 ° C, as can be seen in
Fig.1, and the introduction of the left helicity in the choles-teric structure of at least a fraction of LC molecules. After the temperature is decreased below this range, the reflected light intensity is restored to a value that is approximately the
same as that at the beginning of the thermal process. It is checked that the application of an electric field does not in-duce any0shift共Fig.5兲.
Then, the material subjected to an electric field is kept at
T− for a few tens of minutes. We measured the reflected
intensity when the incident beam was right- or left-handed circularly polarized and we found 76% and 53%, respec-tively. However, these quantities consist of the intensity of the light as reflected by the experimental cell and also by the polarizer. After measurement of the intensity reflected by the right- and left-circular polarizers 共43% and 42%, respec-tively兲, we finally found for the intensity reflected by the
0 5 10 15 20 25 30 35 40 45 50 70 75 80 85 90 95 100 Temperature (°C) Transmission losses (%, a . u.)
Without electric field With electric field
FIG. 3. Transmission losses due to light scattering as a function of temperature共monomer concentration=6.2 wt %兲 when an electric field 共91.5 V at 50 kHz兲 is applied from T+共curing temperature兲 to T−共measurement temperature兲 or not.
0 5 10 15 20 25 30 35 40 45 50 70 75 80 85 90 95 100 Temperature (°C) Reflected ligh t in ten sity (%, a. u .)
Without electric field With electric field
FIG. 4. Reflected light intensity as a function of temperature共monomer concentration=6.2 wt %兲 when an electric field 共91.5 V at 50 kHz兲 is applied from
T+共curing temperature兲 to T−共measurement temperature兲 or not.
experimental cell when the incident beam is right or left circularly polarized the following values: IR= 33% and IL = 11%. When the electric field is switched off, the values are
IR= 29% and IL= 7%. This decrease shows that the texture relaxes toward a less homogeneously aligned planar texture. In summary, the application of an electric field to the PSCLC when its temperature is changed from T+ to T− al-lows共i兲 the reduction in the nucleation of a polydomain tex-ture due to drastic pitch changes in the cholesteric structex-ture of the free fraction by promoting a monodomain planar tex-ture and共ii兲 the measurement of the reflected light intensity associated with the two distinct polarizations, which gives evidence to the property of double-handed circularly polar-ized light reflection. This property originates from the two-population structure of low molar mass LCs. Each popula-tion gives rise to a band of circularly polarized light that is selectively reflected. One was influenced to a large extent by the network and participated in the reflection of right-handed circularly polarized light, which corresponds to the reflectiv-ity of the blend when the UV light curing occurred. The second population behaves much the same as in the bulk and participated in the reflection of left-handed circularly polar-ized light, which corresponds to the reflectivity of the blend at the final measurement temperature.
The procedure represents a technological alternative so-lution to the multilayer systems. Indeed, it is well known that two CLC cells with the same mean reflection wavelengths but opposite helicity senses may be stacked to increase the reflected light intensity.14 The cuticle of the beetle Plusiotis
Resplendens also selectively reflects light with a
double-handed circular polarization between 520 and 640 nm.15The chitin molecules of the three-layer cuticle adopt a liquid crystalline structure: a half-wave-plate, made of molecules with a nematic organization, is included between two choles-teric layers of left-handed helicity sense; the right-handed polarized light that is passed through the first layer is
con-verted into left-handed polarized light by the half-wave-plate and finally reflected from the third layer. However, a multilayer solution is typically undesirable due to the optical losses at the interfaces共reflection, scattering兲 and the cost of implementation for applications.
IV. CONCLUSION
A double-handed circularly polarized light band may be reflected by a PSCLC single-layer material. We have found the right balance between the free and bound populations of LC molecules, which is related to the concentration in a network-forming material and, finally, to the geometrical confinement of the LC into the network. Light scattering may arise from a polydomain structure due to the drastic pitch changes in the helical structure of the free population when the temperature is changed from the polymerization temperature to the measurement one. We have found a solu-tion for preserving a monodomain texture and, in return, in-creasing the reflected light intensity by subjecting the PSCLC 共which has a negative dielectric anisotropy兲 to an alternative electric field during the thermal process. Conse-quently, it was possible to discriminate the respective quan-tities of left and right circular polarized components in the total amount of reflected light intensity. Potential applica-tions are related to the light management for smart windows or reflective polarizer-free displays with a larger scale of reflectivity levels and, more generally, for regulation of tem-perature, telecommunications, or stealthiness.
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2.5 2.6 2.7 2.8 2.9 3 3.1 3.2 70 75 80 85 90 95 100 Temperature (°C) Mean reflection wavelength 0 (µm)
Without electric field With electric field
FIG. 5. Mean reflection wavelength as a function of temperature共monomer concentration=6.2 wt %兲 when an electric field 共91.5 V at 50 kHz兲 is applied from T+共curing temperature兲 to T−共measurement temperature兲 or not.
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